There has been a need for sometime for packing materials for use in, for example, fast foods which is considerably less expense than package materials heretofore used and which is biodegradable and repulpable. Considerable efforts have been made in the prior art to provide such packaging materials which have the necessary resistance to grease, but without success.
For example, in U.S. Pat. No. 337,155 there is described a grease-proof paper in which two or more sheets of a treated paper are bonded together to form a solid laminate by means of a sodium silicate adhesive. In the composite there described, the sodium silicate adhesive is laid down as a continuous layer between the sheets of paper, and that type of arrangement does not provide any substantial insulating properties. In addition, the composite described in the foregoing patent does not have the necessary flexibility for use in, for example, sandwich wrappings.
Attempts have been made to improve on that basic concept, typically by including polymer substrates as a layer in the composite. One such example is described in U.S. Pat. No. 5,128,182 which describes a composite formed of a layer of absorbent material and a layer of a printable material with an intermediate layer interposed there between formed of a pigmented polymer. The need to use a polymer layer in the composite increases substantially the cost of such a composite, and, at the same time, minimizes the ability of the composite to undergo biodegrading and repulping. In addition, because of the need to use a polymer layer in the composite, the establishment of a secure bonding relationship between the paper layers is also made more difficult. A similar packaging material is described in U.S. Pat. Nos. 2,518,762 and 2,526,787.
Other attempts to develop packaging materials are exemplified by U.S. Pat. No. 5,023,134 which utilizes a paper substrate having a polypropylene-base wax coating on each side of the paper. Composites of that sort do not provide the necessary insulating properties frequently desired for packaging applications.
It has been proposed in U.S. Pat. No. 4,487,796 to provide a laminate of absorbent paper sheets by selectively adhering together with a sodium silicate adhesive to absorbent paper products, thus forming spot laminates with air pockets distributed between the sheets. That patent illustrates one of the problems which the prior art has faced in the manufacture of packaging materials. When the paper sheets employed have been chemically treated to render them moisture and grease resistance, there has been a tendency for adhesives to fail in establishing a bonding relationship between the treated paper sheets. In other words, the more resistant the paper sheets are impermeable by either moisture or grease, the more resistance such sheet exhibit toward bonding each to the other.
Accordingly, it is an object of the present invention to produce and provide a method for producing a packaging material suitable for use as a sandwich wrap in which two or more sheets of paper which have been treated is moisture and grease resistance are bonded together to provide an inexpensive, biodegradable and repulpable paper product.